In semiconductor processing, a large portion of the yield losses can be attributed to contaminations by particles and films of various nature. The contaminants may be organic or inorganic particles, films formed of polymeric bases, or other ionic based materials. The particles or films may be generated as byproducts in the reaction of reactant gases, by the surrounding environment, by the processing equipment or by the handling of manufacturing personnel. Some contaminants are particles or films generated from condensed organic vapors, solvent residues, photoresist or metal oxide compounds.
Typical problems and the detrimental effects caused by particle or film contaminants are poor adhesion of deposited layers, poor-formation of LOCOS oxides, or poor etching of the underlying material. The electrical properties and the stability of devices built on the semiconductor substrate may also be seriously affected by ionic based contaminants. The various forms of contaminants therefore not only reduce the product yield but also degrade the reliability of the devices built. For instance, contaminant particles can cause a device to fail by improperly defined patterns, by creating unpredictable surface topography, by inducing leakage current through insulating layer, or otherwise reducing the device lifetime. It is generally recognized that a particle contaminant that exceeds one-fifth to one-half of a minimum feature size on a device has the potential of causing a fatal defect, i.e. a defect that causes a device to fail completely. A defect of smaller size may also be fatal if it falls in a critical area, for instance, a particle contaminant in the gate oxide layer of a transistor. In modern high density devices, such as a dynamic random access memory chip, the maximum allowable number of particle contaminants per unit area of the device must be reduced accordingly in order to maintain an acceptable yield and reliability.
One of the widely used processing techniques for semiconductor wafers is a low pressure chemical vapor deposition (LPCVD) technique. A LPCVD process can be carried out in an apparatus such as that shown in FIG. 1. The LPCVD method has been widely used in the deposition of silicon nitride or TEOS oxide films on semiconductor wafers. In the method, a gas containing the structural elements of the film material to be formed is first fed into a chamber, followed by heating the gas mixture to induce a chemical reaction to deposit the desired film on the semiconductor substrate. In a conventional CVD method, a silicon nitride film can be deposited by a chemical reaction between silane (SiH.sub.4) and ammonia (NH.sub.3) at 1 ATM and a temperature of 700.about.900.degree. C., or by a mixture of dichlorosilane (SiCl.sub.2 H.sub.2) and ammonia at a reduced pressure and a temperature of 700.about.800.degree. C.
As shown in FIG. 1, reactant gases of dichlorosilane 12 and ammonia 14, each carried by a carrier gas of nitrogen, are fed into the process chamber 18 through the inner tubes 40. The reaction gases are mixed at the bottom portion of inner tubes 40. Manifold 16 provides inlets and outlets for the gases and is used as a pedestal support for the inner tubes 40 and the outer tubes 24. The process chamber 18 is first evacuated by vacuum pump 20 prior to the reaction. A purge gas of nitrogen 22 is then used to fill the process chamber 18 and to drive out any residual gas left from the previous deposition cycle. A cold trap 26 maintained at sub-ambient temperature, e.g., of approximately 12.about.18.degree. C., is used in the vacuum line to trap particles that cannot be pumped away. The manifold 16 is provided with a pressure sensor 28 which is connected via a conduit 30 to the manifold 16 at a pressure port 32. A main valve 34 and pressure switches (not shown) are provided in the vacuum evacuation line for controlling the fluid flow. A vent line 48 is connected to the vacuum evacuation line for venting spent reactant gases through control valves 52 and 54 to the exhaust vent 56.
After the reactant gases of SiCl.sub.2 H.sub.2 and NH.sub.3 are mixed in the inner tube 40, the gas mixture is flown into the process chamber 18 to deposit silicon nitride films on wafers held in a wafer boat (not shown). It has been observed that during the reaction between SiCl.sub.2 H.sub.2 and NH.sub.3, a reaction byproduct of NH.sub.4 Cl is frequently produced. The ammonium chloride powder which is in a very fine powdery form causes a defect on the wafer surface known as nitride haze. It is believed that during a nitride deposition process, contaminating powder may be coated inside the conduit between the chamber 18 and the cold trap 26, inside the conduit between the cold trap 26 and the gate valve 34, inside the gate valve 34, and inside the conduit between the gate valve 34 and the automatic pressure controller 20. The powdery contaminant may then be siphoned back into the process chamber 18 during an unintentional back-flow process. The fine powder of ammonium chloride deposits on top of a wafer surface and forms a haze defect. The nitride haze, once formed, is very difficult to remove from the wafer surface. For instance, a wet scrubbing method by using a brush cannot remove the haze from the wafer surface. The nitride haze acts as an additional insulating layer on top of the silicon wafer and presents processing difficulties in subsequently carried out processes. One of such processing difficulties occurs in the formation of LOCOS oxide insulation. The nitride haze impedes the growth of LOCOS oxide. Similar contaminants have also been observed in a TEOS oxide deposition process with similarly undesirable results.
In an effort to reduce or eliminate the nitride haze problem, a bypass vent pipe has been used to bypass the gate valve and to provide continuous pumping of the chamber during wafer loading and unloading steps. Even though this method reduces somewhat the magnitude of the chamber contamination problem, the small vent tube used for bypassing the gate vale is frequently plugged with the contaminating particles. The cleaning of such tubes becomes a time and labor consuming process during a preventive maintenance procedure. It is therefore desirable to have a bypass vent pipe for use in such application that does not get plugged up and furthermore, it would be desirable to have a vent pipe that is capable of indicating when such blockage occurs so that the vent pipe may be serviced.
It is therefore an object of the present invention to provide a closed-loop controlled apparatus for preventing contamination to a low pressure chemical vapor deposition chamber that does not have the drawbacks or shortcomings of the conventional apparatus of a vent tube.
It is another object of the present invention to provide a closed-loop controlled apparatus for preventing contamination to a low pressure chemical vapor deposition chamber that utilizes an exhaust vent for bypassing a gate valve such that the chamber may be continuously pumped during wafer loading and unloading to prevent a back-flow of contaminating particles into the chamber.
It is a further object of the present invention to provide a closed-loop controlled apparatus for preventing contamination to a low pressure chemical vapor deposition chamber by utilizing an exhaust vent equipped with a reduced cross-sectional area such that a fluid flow rate through the area is significantly higher than through the other areas of the vent in order to prevent the cumulation of contaminating particles in the exhaust vent.
It is another further object of the present invention to provide a closed-loop controlled apparatus for preventing contamination to a low pressure chemical vapor deposition chamber which includes an exhaust vent that has conduits of large cross-sectional area connected by a conduit of small cross-sectional area such that any cumulation of a fine powdery ammonium chloride material in the exhaust vent can be avoided.
It is still another object of the present invention to provide a closed-loop controlled apparatus for preventing contamination to a low pressure chemical vapor deposition chamber which includes an exhaust vent that is equipped with a large diameter cross-sectional area and a butterfly valve contained therein for adjusting the fluid flow rate through the conduit.
It is yet another object of the present invention to provide a closed-loop controlled apparatus for preventing contamination to a low pressure chemical vapor deposition chamber which includes an exhaust vent that is equipped with a butterfly valve for controlling a fluid flow rate through the vent and for feeding back a signal to a controller such that the operation of the deposition chamber can be shut off when the angle of the butterfly valve exceeds a pre-set value.
It is still another further object of the present invention to provide a closed-loop controlled method for preventing contamination to a low pressure chemical vapor deposition chamber incorporating the steps of connecting an exhaust vent to the process chamber and equipping a conduit of the vent with a butterfly valve for adjusting a fluid flow rate through the conduit.
It is yet another further object of the present invention to provide a closed-loop controlled method for preventing contamination to a low pressure chemical vapor deposition chamber that utilizes an exhaust vent with a butterfly valve installed therein for adjusting a fluid flow rate and sending out a signal to a controller such that the operation of the deposition chamber can be shut off when the angle of the butterfly valve detected exceeds a pre-set value.